In today's wireless communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A wireless communications network comprises network nodes, i.e. base stations or radio base stations, providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, are served in the cells by the respective network node and are communicating with respective network node. The wireless devices transmit data over an air or radio interface to the network nodes in uplink, UL, transmissions and the network nodes transmit data over an air or radio interface to the wireless devices in downlink, DL, transmissions.
Long Term Evolution, LTE, is a project within the 3rd Generation Partnership Project, 3GPP, to evolve the WCDMA standard. LTE provides advantages such as increased capacity, higher data peak rates and significantly improved latency. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex, FDD, and Time Division Duplex, TDD, operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing, OFDM, is used in a DL transmission from a network node to a wireless device. Single Carrier-Frequency Domain Multiple Access, SC-FDMA, is used in an UL transmission from the wireless device to the network node. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread, DFTS-OFDM.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element, RE, corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix, cp, which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0 -#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
DL and UL transmissions are dynamically scheduled. For example, in each DL subframe, the network node transmits control information about to or from which wireless device data is transmitted and upon which resource blocks the data is transmitted. The control information for a given wireless device is transmitted using one or multiple Physical Downlink Control Channels, PDCCH. Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe, where n is the Control Format Indicator, CFI. Typically the control region may comprise many PDCCHs carrying control information to multiple wireless devices simultaneously. Similarly, in each uplink subframe, the wireless device may transmit control information using one or multiple Physical Uplink Control Channels, PUCCHs.